Salt contamination occurs in hot countries where excessive land clearance and/or irrigation have taken place... once the native trees are cut down, the grasses that farmers tend to grow as crops, such as wheat, often fail to release water into the atmopsphere as fast as it is being pulled up to the surface by the sun... this leads to surface waterlogging (which drowns plants) - irrigation speeds up the waterlogging step - eventually the water table is raised to such an extent that it becomes impossible to drain the land and prevent the water as well as the salts it carries (which have been washed deep underground by millions of years of rain) from reaching the surface... over many decades the water-borne salts precipitate out at the surface (due to evaporation) and become highly concentrated... eventually, even salt-tolerant plants stop being able to grow + the land turns into desert....